These are the four Galilean satellites. They are named so
because they were the only four moons that Galileo was
able to see. They are also the largest of Jupiter's moons. From top-left
in a clock-wise direction, they are Ganymede, Callisto, Europa, and Io.

If Ganymede were not bound to Jupiter, it would be considered
a planet in its own right. It is actually bigger than Mercury,
being 390 km (234 miles) larger in diameter. It has heavily cratered dark
regions, with lighter expanses in-between. Geologists think that it used
to have plates, like the Earth, but they froze together
soon after Ganymede's birth.

Callisto, the outermost of the Galilean moons, is almost
an exact twin of Mercury in size and appearance. Every square mile is covered
with craters or other signs of bombardment. Other than that, there are no
distinct characteristics

Europa, closer to Jupiter than Ganymede, is the smoothest
natural body in the solar system. It resembles a billiard ball until seen
very close-up. At that distance you can start to see dark, deep, and narrow
cracks. In scale, though, the relief is no bigger than a line on a billiard
ball made with a felt-tipped marker. Geologists think that Europa has liquid
water underneath the icy surface - and possibly life.

Io, closer yet, is commonly compared to a pizza. Its volcanoes
make it the most active world in the solar system. They spew out the sulfuric
acid that gives Io its many colors. They also make Io one of the only three
moons with an atmosphere in the solar system. Saturn's Titan and Neptune's Triton are
the other two moons. Io is similar in size and composition to our
moon. Io is caught in the middle of a tug-of-war between Jupiter and
the other moons. Tension has melted the interior and raised the surface temperatures
so high that scientists calculated that it generates the most heat for its
size of any body in the solar system, except for the sun.

Other Moons

There are 59 other moons that have been discovered around
Jupiter. All of them are smaller than the Galilean Satellites, and most of
them are the size of small asteroids. Most have highly elliptical orbits
and are highly inclined relative to Jupiter's equator. These are all very
strong indications that the moons are not native to Jupiter, but rather they
were captured. The largest piece of evidence for this is that over 80% of
the non-Galilean Satellites orbit retrograde to
Jupiter's spin.

The largest non-Galilean moon is Amalthea (right; image
is a three-color composite taken by Voyager 1), being 262 km (163 miles)
at its widest axis.

The innermost two moons, Metis and Adrastea, patrol the
outer edge of the rings, their small gravity being enough to keep the particles
from flying out. There are two other moons that are closer to Jupiter than
Io; they are Amalthea and Thebe.

Nearly all of the moons that have been discovered in the
last few years orbit retrograde to the direction of Jupiter's rotation ,
indicating that they are most likely not native to the system. They are also
very small. Those that have not yet been given names will eventually be given
names by the International Astronomical
Union, the only official naming group.

Data for Jupiter's Moons

Name

Discovery
Date

Discoverer

Distance
from Jupiter (103 km)

Orbital
Period (days)

Inclination

Eccentricity

Mass (1020 kg)

Radius
(km)

Geometric
Albedo

Io (JI)

1610

Galileo Galilei

421.6

1.769138

0.04

0.004

893.2

1821.6

0.62

Europa (JII)

1610

Galileo Galilei

670.9

3.551181

0.47

0.0101

480.0

1560.8

0.68

Ganymede (JIII)

1610

Galileo Galilei

1070.4

7.154553

0.21

0.0015

1481.9

2631.2

0.44

Callisto (JIV)

1610

Galileo Galilei

1882.7

16.689018

0.51

0.007

1075.9

2410.3

0.19

Metia (JXVI, S/1979 J3)

1979

S. Sunnott

128.0

0.294779

0.06

0.0002

0.001

20

0.06

Adrastea (JXV, S/1979 J1)

1979

Jewitt and Danielson

129.0

0.298260

0.03

0.0015

0.0002

13 x 10 x 8

0.10

Amalthea (JV)

1892

E. Barnard

181.4

0.498179

00.40

0.003

0.075

131 x 73 x 67

0.09

Thebe (JXIV, S/1979 J2)

1979

S. Synnott

221.9

0.6745

0.8

0.018

0.008

55 x 45

0.05

Themisto (JXVIII, S/1975 J1)

1975

7507

130.02

45.67

0.242

4

0.04

Leda (JXIII)

1974

C. Kowall

11170

240.92

27.47

0.164

0.00006

5

0.07

Himalia (JVI)

1904

C. Perrine

11460

250.5662

27.63

0.162

0.095

85

0.03

Lysithea (JX)

1938

S. Nicholson

11720

259.22

27.35

0.112

0.0008

12

0.06

Elara (JVII)

1905

C. Perrine

11740

259.6528

24.77

0.217

0.008

40

0.03

S/2000 J11

2000

12560

287.0

28.3

0.248

2.0

0.04

Euporie (JXXXIV, S/2001 J10)

2001

19390

553.1*

147.0

0.156

1.0

Euanthe (JXXXIII, S/2001, J7)

2001

21030

620.0*

145.9

0.176

1.5

Harpalyke (JXXII, S/2000 J5)

2000

21110

623.3*

148.7

0.227

2.2

0.04

Praxidike (JXXVII, S/2000 J7)

2000

21150

625.3*

148.7

0.220

3.4

0.04

Orthosie (JXXXV, S/2001, J9)

2001

21170

623.0*

141.9

0.272

1.0

Iocaste (JXXIV, S/2000 J3)

2000

21270

631.5*

159.7

0.218

2.6

0.04

Ananke (JXII)

1951

S. Nicholson

21280

629.8*

148.9

0.244

0.0004

10

0.06

Hermippe (JXXX, S/2001 J3)

2001

21250

631.9*

150.3

0.251

2.0

Thyone (JXXIX, S/2001 J2)

2001

21310

632.4*

149.0

0.295

2.0

Arche (JXLIII, S/2002 J1)

2002

22930

723.9*

165.0

0.259

1.5

Pasithee (JXXXVIII, S/2001 J6)

2001

23030

716.3*

165.4

0.288

1.0

Kale (JXXXVII, S/2001 J8)

2001

23120

720.9*

165.3

0.475

1.0

Chaldene (JXXI, S/2000 J10)

2000

23180

723.8*

165.4

0.238

1.9

0.04

Isonoe (JXXVI, S/2000 J6)

2000

23220

725.5*

165.0

0.261

1.9

0.04

Eurydome (JXXXII, S/2001 J4)

2001

23220

720.8*

150.1

.0345

1.5

Erinome (JXXV, S/2000 J4)

2000

23280

728.3*

164.9

0.270

1.6

0.04

Taygete (JXX, S/2000 J9)

2000

23360

732.2*

165.2

0.251

2.5

0.04

Carme (JXI)

1938

S. Nicholson

23400

734.2*

164.9

0.253

0.001

15

0.06

Kalyke (JXXIII, S/2000 J2)

2000

23580

743.0*

165.2

0.243

2.6

0.04

Aitne (JXXXI, S/2001 J11)

2001

23550

741.0*

165.7

0.291

1.5

PasiphaÅ½ (JVIII)

1908

P. Melotte

23620

743.6*

151.4

0.409

0.003

18

0.10

Megaclite (JXIX, S/2000 J8)

2000

23810

752.8*

152.7

0.425

2.7

0.04

Sponde (JXXXVI, S/2001 J5)

2001

23810

749.1*

155.0

0.454

1.0

Sinope (JIX)

1914

S. Nicholson

23940

758.9*

158.1

0.250

0.0008

14

0.05

Callirrhoe (JXVII, S/1999 J1)

1999

Spacewatch Project Minor Planet Center

24100

758.8*

147.1

0.283

4

0.04

Autonoe (JXXVIII, S/2001 J1)

2001

24120

765.1*

151.9

0.415

2.0

Eukelade (XLVII, S/2003 J1)

2003

24560

781.6*

163.4

0.345

4.0

Helike (XLV, S/2003 J6)

2003

20980

617.3*

156.1

0.157

4.0

Aoede (XLI, S/2003 J7)

2003

23810

748.8*

159.4

0.405

4.0

Hegemone (JXXXIX, S/2003 J8)

2003

24510

781.6*

152.6

0.264

3.0

Kallichore (XLIV, S/2003 J11)

2003

22400

683.0*

163.9

0.223

2.0

Cyllene (XLVIII, S/2003 J21)

2003

24000

737.8*

141.0

0.412

2.0

Karpo (XLVI, S/2003 J20)

2003

17100

456.5

55.1

0.295

3.0

Mneme (JXL, S/2003 J21)

2003

20600

599.0*

148.0

0.208

2.0

Thelxinoe (XLII, S/2003 J22)

2003

20700

601.0*

151.1

0.233

2.0

S/2003 J2

2003

28570

982.5*

151.8

0.380

2.0

S/2003 J3

2003

18340

504.0*

143.7

0.241

2.0

S/2003 J4

2003

23260

723.2*

144.9

0.204

2.0

S/2003 J5

2003

24080

759.7*

165.0

0.210

4.0

S/2003 J9

2003

22440

683.0*

164.5

0.269

1.0

S/2003 J10

2003

24250

767.0*

164.1

0.214

2.0

S/2003 J12

2003

19000

533.3*

145.8

0.376

1.0

S/2003 J14

2003

25000

807.8*

140.9

0.222

2.0

S/2003 J15

2003

22000

66.4*

140.8

0.110

2.0

S/2003 J16

2003

21000

595.4*

148.6

0.270

2.0

S/2003 J17

2003

22000

690.3*

163.7

0.190

2.0

S/2003 J18

2003

20700

606.3*

146.5

0.119

2.0

S/2003 J19

2003

22800

701.3*

162.9

0.334

2.0

S/2003 J23

2003

24060

759.7*

149.2

0.309

2.0

* This indicates that the moon orbits in a retrograde -
the opposite direction to the planet's spin - motion.